EXCEED THE SPACE PROVIDED. Memory CDST cells play a critical role in mediating protective immunity against infectious disease. Generating long- lived memory CDS T cells is currently an important goal of many vaccines being developed to fight chronic viral infections, such as HIV and HCV, and also against certain types of cancers. Memory CDS T cells provide long-lived immunological protection because they proliferate, secrete antiviral cytokines and kill infected cells more rapidly than naive T cells upon antigen encounter, and also they can persist for great lengths of time-often for decades or longer. The specific aims of this proposal are as follow: 1. To understand the mechanisms that regulate the development of memory CDS T cells. For many years, a central pursuit in the field of memory T cell development has been to identify the population of antigen specific effector CDST cells that has the potential to develop into memory cells. We have recently succeeded in this by showing that CD127 (IL-7Ra) expression distinguishes between the effector cells that will die versus the ones that will survive to form long- lived memory cells. This breakthrough has now positioned us well to address mechanistic questions about memory T cell development. 2. To characterize memory CDS T cell proliferation in Ivmphoid versus non-lvmphoid tissues and to identify the major sites of memory T cell division. It is now well established that memory CDS T cells are present in both lymphoid and non-lymphoid tissues and it is also well established that IL-15 mediated homeostatic proliferation of memory CDST cells is essential for maintaining long-term immunity. However, it is not known whether homeostatic proliferation occurs in all tissues or whether there is a preferential site of memory T cell turnover. Our preliminary results suggest that the bone marrow may be a preferred site of memory T cell division. Experiments will be done to confirm and further extend these observations. Of particular interest will be identifying the soluble and/or cellular interactions specific to the bone marrow that regulate memory T cell homeostasis. 3. To determine the quality of vaccine-induced memory CDST cells. We have initiated experiments to examine the quality of memory CDS T cells induced by a live acute viral infection compared to immunization with DNA or virus-like- particles (VLP). We found that DNA or VLP induced memory CDS T cells were not as efficient as LCMV induced memory cells in terms of their ability to make a recall response and confer protective immunity. Interestingly, DNA or VLP induced memory CDS T cells were also compromised in their ability to persist long-term and to undergo homeostatic proliferation. Experiments will be done to characterize in detail the process of memory T cell differentiation after vaccination with DNA or VLP and to understand why DNA or VLP-induced memory CDS T cells are not as "fit" as the memory cells generated after a live infection. An important goal of these studies is to develop strategies for improving the quality of memory CDS T cells induced by non-replicating and safer vaccines.